This invention relates to weather radar, aircraft runaway incursions, and specifically for a weather radar for the detection and alerting of the presence of obstacles during taxi operations.
Accidents during surface operations remain one of the last largely unsolved problems of the air transport system. Both the US government and resources from a number of companies are moving to meet this challenge. Many have suggested solutions that require controlled environments, ground equipment, or equipment on all vehicles or structures that may be involved in an accident. Counting on these external requirements always being in place insures some situations will never be covered, even in a regulatory rich environment such as North America. Second and third world operations with their historically poorer infrastructure and minimally equipped aircraft will see a much higher rate of situations where an aircraft that is equipped with hazard detecting equipment that requires external help may still have an avoidable accident. The real challenge is to make an aircraft safe in all environments. This forces at least some portions of the system to be autonomous.
A representative system under development is a ground-based system that requires either surface movement radar and/or multilateration receivers processing L-band returns from aircraft. Multilateration is a process of solving for the position of a transmitter on board an aircraft by measuring the transmitter pulse arrival time to geographically diverse sets of ground-based receivers whose positions are known. Since this system is ground based, it does not allow the aircraft to operate autonomously. Secondly, since this multilateration system tracks targets by L-band emissions, targets without a transponder are not detectable.
NASA is developing a system called RIPS (Runway Incursion Prevention System) that is based on ADS-B (automatic dependent surveillance-broadcast) equipped aircraft, an airport database, and an L-band data path. Both obstacle awareness and position awareness are provided with this system. Although this system can be carried on an aircraft, targets to be avoided must cooperate by squittering their position at L-Band. Any object without GPS position and an L-band transmitter will not be tracked.
FLIR (forward looking infrared) cameras have been tested with both single and multiple IR bands. This solution does not require cooperation from a viewed target but does not work in some fog situations. Millimeter wave systems have been demonstrated from several vendors but again they may be challenged with fog and cloud cover in the passive systems while the active systems are heavily attenuated in rain. Both the IR and millimeter wave systems are costly and also carry a new maintenance burden as airline operators try to maintain the new higher frequency equipment and new IR window or dual-band millimeter wave capable radomes.
Another representative system is an add-on to an enhanced ground proximity warning system (EGPWS). This system produces database driven audio annunciation of impending transitions of runways and taxiways. This system's strong point is improving the pilot's awareness of own aircraft location. Thus the system is useful in helping an equipped aircraft from making a mistake that causes an accident. This system's weak point is in only annunciating runway/taxiway transitions, it does not directly detect threats. Thus this system will not protect the aircraft from someone else who makes a mistake or obstacles that are not contained in the database. In addition this system frequently annunciates with an audio message whether a threat exists or not. This approach will violate the dark/quiet cockpit philosophy and may lead the pilot to discount an accident avoiding annunciation since the system is producing annunciations that are discounted all the time.
The aircraft ground movement problem can be separated into two components, position awareness and threat awareness. Position awareness aids the pilot in not putting his aircraft in the wrong place where it can be threatened or become a threat to another aircraft. Threat awareness is looking out for other aircraft and obstacles that are in a wrong or unexpected position. The EPGWS-based system described above is for improved position awareness. A HUD-driven taxi guidance system is proposed to keep the aircraft where it should be. Assuming a guidance or position awareness system, such as these, is included in new aircraft, the problem becomes one of detection and annunciation of threats.
Threat detection solutions fall into two broad categories; cooperative and non-cooperative. Cooperative detection of hazards requires that a potential hazard be actively involved in the detection process. Examples include the L-band multilateration driven system and the GPS position annunciating NASA RIPS system previously described. On the other hand, the FLIR, radar, and millimeter wave systems do not require cooperation from a target.
Threat detection systems may be evaluated by their detection rate, false alarm rate, and accuracy of the threat prediction. A cooperative system's miss rate should be dominated by the number of obstacles that for any reason do not cooperate as desired. Unintended objects on the runways that were never expected to cooperate or aircraft with missing or broken equipment are examples of obstacles that would produce misses. Assuming all obstacles are equipped to cooperate, very high detection rates are achievable and this will aid certification of a cooperative system. Non-cooperative systems have their miss rate dominated by their detection miss rate. High detection rates for non-cooperative systems come at the cost of false alarms. This may make certification somewhat of a challenge for any non-cooperative sensor system unless the environmental and target conditions are limited during certification as was done in certification standards for windshear detecting weather radar.
Both cooperative and non-cooperative systems face challenges. Since a RIPS or HUD taxi guidance system can provide taxi guidance and position awareness along with low installed weight and cost, it is likely to be part of a solution. So the problem that remains is that of obstacles and aircraft not being equipped to cooperate or not in the current database. A non-cooperative system can fill this detection void.
Non-cooperative threat sensors include cameras (visual, low light, millimeter wave, and infrared) and radars (microwave, millimeter wave, and laser). As previously discussed, cameras, whether low light, infrared, millimeter wave, or visual, all fail under some precipitation situations that pilots would call benign. Both millimeter and laser radars do not work in moderate to high rain rate situations. An ideal sensor would be low cost or already on the aircraft and work in all environmental conditions.
Ground movement threat assessment may be broken into two problems, short-range obstacles and longer range moving obstacles. The short-range problem is one of providing the pilot with timely information to allow the aircraft to be braked to a stop. Short-range targets may be either moving or non-moving. The inclusion of non-moving targets brings a potentially higher false alarm opportunity unless a database is used to qualify which targets are in the taxi path of the aircraft. The longer-range problem only addresses moving targets. Targets at range that are not closing are not a threat.
What is needed is a reliable low-cost autonomous system for detection and alerting of the presence of obstacles during taxi operations under all weather conditions.